Although the present invention has general applicability to any given aqueous system where the formation and deposition of scale and in particular calcium carbonate scale is a potential problem, the invention will be discussed in detail as it concerns systems encountered in the paper industry. In addition to the paper industry, scale formation and deposition may be problematic in aqueous systems such as boiler water systems, cooling water systems and gas scrubber systems.
In paper manufacturing facilities the formation and deposition of scale forming materials can cause quality and efficiency problems in the pulp and papermaking systems. Calcium carbonate is a common scale in kraft digesters, green liquor lines and bleach plant extraction stages. Kraft processing is a predominant pulping method due to its efficient recovery process for the cooking chemicals. In a kraft process the cooking solution is known as white liquor and contains NaOH and Na.sub.2 S. The white liquor is used to cook wood chips in a digester. In the cooking process, lignin, which binds the wood fiber together is dissolved in the white liquor forming pulp and black liquor. Upon separation, the black liquor is concentrated and burned in a recovery furnace to provide power and allow the recycle of chemicals. In burning black liquor an inorganic smelt of Na.sub.2 CO.sub.3 and Na.sub.2 S is formed. This smelt is dissolved in water to form green liquor which is reacted with quick lime (CaO) to convert Na.sub.2 CO.sub.3 into NaOH and regenerate white liquor. This conversion is typically 85-95% efficient leaving a small percentage of Na.sub.2 CO.sub.3 in the white liquor.
In the digester where wood chips are cooked with white liquor, calcium is leached from the wood or is present in the recycled chemicals along with Na.sub.2 CO.sub.3 and other sodium salts. At low temperatures lignin is a good complexing agent for these materials but at higher temperatures these complexes can decompose during the cooking process giving rise to free calcium ions. In the separation of black liquor from the pulp, filter screens can be contaminated with scales such as CaCO.sub.3 which restrict the flow of the black liquor.
In the formation of green liquor by evaporating, oxidizing and forming a smelt from the black liquor, the formation of scale is possible when the smelt is dissolved in water to form green liquor. Scale, such as calcium carbonate scale can restrict or block the flow in green liquor lines.
In a bleach plant extraction stage chlorinated and oxidized lignin is removed by dissolution in a caustic solution. This process is typically carried out at temperatures between 60.degree. and 80.degree. C. and a final pH of about 10.8. Calcium ions, which would give rise to calcium carbonate scale formation, are less likely where bleaching virgin pulp since it would have been removed in earlier stages. However, when recycled paper is used in forming the pulp, CaCO.sub.3 from coatings or fillers can cause the formation of CaCO.sub.3 scale.
Deposit control agents such as phosphates, phosphonates and polyacrylates are often used to inhibit calcium carbonate scale formation in industrial cooling water systems. The use of polyacrylate alone is not effective at high calcium concentrations because undesirable polyacrylate--calcium adducts are formed reducing efficacy. Although phosphonates are very effective at controlling carbonate scale formation, they can produce insoluble phosphonate--calcium complexes or calcium phosphate scale upon degradation. Further, current limits on phosphate discharge limit the acceptability of the use of phosphonates for water treatment. Certain phosphonates exhibit excellent calcium tolerance, that is the ability to inhibit calcium carbonate scale in water having a propensity toward scale deposition.
Preventing the corrosion and scaling of pulp and paper manufacturing equipment is essential to the efficient and economical operation of such systems. Excessive corrosion of metallic surfaces can cause the premature failure of process equipment, necessitating downtime for the replacement or repair of the equipment. Additionally, the buildup of corrosion products on heat transfer surfaces impedes water flow and reduces heat transfer efficiency, thereby limiting production or requiring downtime for cleaning. Reduction in efficiency will also result from scaling deposits which retard heat transfer and hinder water flow.
Scale can also cause flow restrictions or rapid localized corrosion and subsequent penetration of metallic surfaces through the formation of differential oxygen concentration cells. The localized corrosion resulting from differential oxygen cells originating from deposits is commonly referred to as "underdeposit corrosion". Therefore, effective control of scaling will also materially affect corrosion.
Methods of inhibiting the formation and deposition of scale imparting compounds in aqueous systems comprising treating the system with a polyepoxysuccinic acid are disclosed in U.S. Pat. Nos. 5,062,962 and 5,147,555. Japanese patent publication 4-166298 discloses methods for preventing metal corrosion and scale generation in aqueous systems by adding polyepoxysuccinic acid or its salts. U.S. Pat. No. 5,344,590 discloses a method for inhibiting corrosion of metal using one or more polytartaric acid compounds (also known as polyepoxysuccinic acids) of a specific molecular weight range. A process for preventing corrosion and the formation of scale in water conducting systems comprising adding one or more phosphonocarboxylic acids or their water soluble salts and optionally other corrosion inhibitors is disclosed in U.S. Pat. No. 3,933,427. U.S. Pat. No. 5,256,332 discloses a method of inhibiting corrosion in aqueous systems. The method comprised adding to an aqueous system a blend of effective amounts of orthophosphate, a polyepoxysuccinic acid, a water soluble azole compound and a copolymer of acrylic acid and an allyl hydroxy propyl sulfonate ether monomer.